I think "Tainted Love" is on here because it is, like the Motown catalogue, now owned by Universal Music; AVI Records Inc. was bought by MCA Records in about 1997/1998, and then PolyGram, which owned Motown, was bought by MCA Universal in 1998/1999, putting both AVI and Motown in Universal's hands. I wouldn't have thought that AVI Record Productions would still exist as a result of that purchase; by the same token, the copyright credit to Motown Record Corporation is incorrect, as the record company became Motown Record Company, L.P. when acquired by MCA in late 1988, and by 2005 had become "Motown Records, a division of UMG Recordings, Inc.". I believe that the original Motown Record Corporation became The Gordy Company in 1988, and I think is now West Grand Media.
Just seen your query RogerFoster - the information with the CD set says the track is copyright 1964 AVI Record Productions Ltd. It doesn't quite answer your question as to how Gloria got herself into this collection but it's an early recording. (All other tracks are copyright to Motown Record Corporation, as you would expect).
Curious to see that Gloria Jones has a version of "Tainted Love" on this at 2-18. Is this her original recording from 1965 or some re-make? As the "Don't Forget The Motor City" website has no mention of her recording a version of "Tainted Love" during her time at Motown it seems to be an interloper on this collection.